This invention relates to an image sensor comprising an array of light responsive pixels. The invention is particularly concerned with focusing of an image on to the array of pixels for a contact-type image sensor.
For a focused image to be formed on a pixel array of a contact-type image sensor, it is possible either to position the original document at a very short distance from the array of pixels, or to provide an optical system for focusing.
If no optical system is provided, the document to be imaged should preferably be spaced from the array of pixels by a distance which is no greater than the pitch of the pixels, for example. If there is greater spacing of the document from the array of pixels, each pixel may receive light from a greater area of the document to be imaged than is desired. As the resolution of an image sensor is increased, the image sensing pixels are arranged with progressively smaller pitch, so that for high resolution image sensors it is not possible to maintain sufficiently small spacing between the document and the array of pixels, and the need arises for an optical focusing arrangement.
U.S. Pat. No. 5,286,605 discloses a solid-state imaging device having a microlens array provided over the image sensor array, each microlens element being associated with an individual pixel of the imaging device. Of course, small microlens are required which must be accurately aligned with the pixels of the image sensor.